The present invention relates to a control system for four-wheel drive vehicle having a continuously variable transmission, and more particularly to a system for controlling a transfer clutch for establishing the four-wheel drive.
Japanese Patent Application Laid Open 60-64035 discloses a four-wheel drive vehicle of the type in which a transfer clutch is provided for transmitting the output torque of a transmission to auxiliary drive wheels. In such a vehicle, a power transmission train can be converted from a two-wheel drive system to a four-wheel drive system by engaging the transfer clutch in accordance with various driving conditions such as rapid start, rapid acceleration or deceleration of the vehicle and skidding. The driving conditions are detected by the difference between rotational speeds of front and rear wheels, depressions of an accelerator pedal and brake pedal and position of a shift lever.
In such a vehicle, the transfer clutch is provided with a hydraulic control circuit having a solenoid operated two-way valve and a transfer control valve. Relationships between duty ratio of the pulses applied to a solenoid of the valve and control pressure P.sub.C applied to the transfer control valve, and quantity Q of drain from the solenoid operated valve are shown in FIG. 7, and a relationship between the duty ratio and clutch torque of the transfer clutch is shown in FIG. 8. Namely, when the duty ratio is 100%, drain quantity Q is maximum and control pressure P.sub.C is at a minimum value. Accordingly, a transfer clutch pressure applied to the transfer clutch becomes minimum, thereby rendering the transfer clutch torque minimum to disengage the clutch, for providing the two-wheel driving.
In a control system for a known continuously variable transmission disclosed in Japanese Patent Application Laid Open 61-105352, even if the transmission ratio is quickly changed, amount of oil supplied to servo devices of pulleys are controlled so as not to exceed a volumetric displacement of a pump, thereby preventing slipping of a belt on the pulleys. In the system, when the engine speed is in a low range, the displacement of the pump is small. Moreover, as shown in FIG. 7, when two-wheel driving mode is selected, a pulse train at a duty ratio 100% is applied to the solenoid operated valve so that the drain quantity Q is at maximum. Accordingly, if the two-wheel driving is selected at low engine speed, the amount of oil supplied to the transfer clutch is insufficient for establishing the two-wheel driving. Consequently, a pump having a large capacity must be installed.